As one type of field-effect transistor, a thin film transistor whose channel region is formed using a silicon film which is formed over a substrate having an insulating surface is known. Techniques in which amorphous silicon, microcrystalline silicon, or polycrystalline silicon is used for the silicon film which is used for the channel region of the thin film transistor have been disclosed (see Patent Documents 1 to 5). A typical application of the thin film transistor is a liquid crystal television device, in which the thin film transistor is practically used as a switching transistor in each pixel in a display screen.
A thin film transistor whose channel region is formed using an amorphous silicon film has problems of low field-effect mobility and low on-state current. On the other hand, a thin film transistor whose channel region is formed using a microcrystalline silicon film has a problem in that, though the field-effect mobility is improved, the off-state current is higher as compared to that of the thin film transistor whose channel region is formed using an amorphous silicon film and thus sufficient switching characteristics cannot be obtained.
A thin film transistor whose channel region is formed using a polycrystalline silicon film features in that the field-effect mobility is far higher and the on-state current is higher than those of the above-described two kinds of thin film transistors. These features enable this kind of thin film transistor to be used not only as a switching transistor in a pixel but also as an element of a driver circuit that needs to drive at high speed.
However, a manufacturing process of the thin film transistor whose channel region is formed using a polycrystalline silicon film involves a crystallization step for a silicon film and has a problem of higher manufacturing costs, as compared to a manufacturing process of the thin film transistor whose channel region is formed using an amorphous silicon film. For example, a laser annealing technique necessary in the process for forming a polycrystalline silicon film has a problem in that large-screen liquid crystal panels cannot be produced efficiently because the area capable of being irradiated with a laser beam is small.
The size of a glass substrate for manufacturing display panels has grown in the following ascending order: the 3rd generation (550 mm×650 mm), the 3.5th generation (600 mm×720 mm or 620 mm×750 mm), the 4th generation (680 mm×880 mm or 730 mm×920 mm), the 5th generation (1100 mm×1300 mm), the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm or 2450 mm×3050 mm), and the 10th generation (2950 mm×3400 mm) The increase in size of the glass substrate is based on the concept of minimum cost design.
However, a technique with which a thin film transistor capable of high-speed operation can be manufactured with high productivity over a large-sized mother glass substrate such as the 10th generation (2950 mm×3400 mm) mother glass substrate has not been established yet, which is a problem in industry.